Diarylnitrones

ABSTRACT

Diaryinitrones are provided which are useful for near UV photolithography.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 836,287, filed Mar. 5, 1986now U.S. Pat. No. 4,859,789, which is a continuation-in-part of ourcopending application Ser. No. 675,918, filed Nov. 28, 1984, which is adivision of abondoned application Ser. No. 536,923, filed Sept. 28,1983, which is a continuation-in-part of abandoned application Ser. No.438,194, filed Nov. 1, 1982 where all of the above applications areassigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION

The present invention is directed to a method of enhancing the contrastof images of objects such as masks for photolithography in themanufacture of integrated circuits and materials therefor.

Lithography in the production of integrated circuits is predominantlycarried out by optical means. In the drive to reduce circuit dimensions,improved performance and increase yield, optical systems have providedthe required resolution with each successive generation of circuittechnology. The image resolution of projection lithographic systems hasrecently begun to approach the physical limits imposed by practicalconstraints on numerical aperture and wavelength. While furtherimprovements in lithographic technology are anticipated, dramaticimprovements in inherent lens resolution are not. In order to continuethe reduction of minimum feature size achievable by optical techniques,it is necessary to alter some other aspect of the lithographic processfor further improvements. One area in which further improvements arepossible is in the photoresist process. Each photoresist ischaracterized by some degree of incident contrast necessary to producepatterns usable for subsequent processing. This minimum requiredcontrast of illumination is referred to as the contrast threshold of theresist. Depending on substrate properties, the required patternthickness and resist edge profiles, conventionally used positivephotoresist has a contrast threshold between 85% and 90% contrast.Currently, most production is done at 90% incident contrast or more. Ifthe contrast threshold of the resist is reduced, the resolutionobtainable with a given optical system is improved due to the fact thatimage contrast is a decreasing function of the spatial frequenciespresent in the image.

The present invention is directed to provide a photoresist process inwhich the contrast of an aerial image utilized in the process isenhanced prior to incidence on the photoresist.

An object of the present invention is to lower the minimum contrastrequired to produce usable images in a photoresist.

Another object of the present invention is to provide newphotobleachable compounds and materials.

In carrying out the invention in an illustrative embodiment thereof, alayer of photoresist of a first thickness and having a predeterminedcontrast threshold is provided. An object or mask is provided havingopaque and transparent areas. An image having a contrast less than thepredetermined contrast threshold of the photoresist is formed of theobject on the layer of photoresist by projecting light of apredetermined wavelength through the object. A layer of photobleachablematerial including a photobleachable compound is provided between theobject and the layer of photoresist and adjacent a surface of the layerof photoresist. The photobleachable compound is sensitive to theaforementioned wavelength of light and has extinction coefficient tomolecular weight ratio in liters per gram-centimeter in the unbleachedstate greater than about 10. The ratio of the extinction coefficient forthe unbleached state to the extinction coefficient for the bleachedstate of the photobleachable material is also greater than about 10.Light of the aforementioned wavelength and of a predetermined intensityis projected through the object onto the layer of photobleachablematerial for a time to obtain a reduction in optical density of thelayer of photobleachable material in direct proportion to the dosage oflight of the aforementioned wavelength incident thereof, whereby theintegrated contrast of the image transmitted by the layer ofphotobleachable material increases with dosage transmitted thereby,reaches a maximum value and thereafter decreases. Parameters of thelayer of photobleachable material are selected such that the maximumvalue of integrated contrast is greater than the predetermined thresholdcontrast of the layer of photoresist. The sensitivity of the layer ofphotoresist and the thickness thereof are selected such that the layerof photoresist is fully exposed by a dosage in a predetermined rangetransmitted by the layer of photobleachable material and provides anintegrated contrast in the transmitted image above the predeterminedcontrast threshold of the photoresist. Light of the aforementionedpredetermined wavelength is projected through the object for a time toprovide the dosage in aforementioned predetermined range transmittedthrough the photobleachable layer. The layer of photobleachable materialis removed and the layer of photoresist is developed whereby a patternrepresenting an enhancement in contrast of the image of reduced contrastof the object is formed in the layer of photoresist.

Another aspect of the present invention is directed to a spin castablemixture capable of forming a photobleachable layer having an absorptionmaximum in the range of 300 to 450 nanometers comprising by weight

(A) 100 parts of organic solvent,

(B) 1 to 30 parts, preferably 5 to 15 parts, of an inert organic polymerbinder,

(C) 1 to 30 parts, preferably 5 to 15 parts, of an aryl nitrone.

The novel features which are believed to be characteristic of thepresent invention are set forth with particularity in the appendedclaims. The invention itself, both as to its organization and method ofoperation, together with further objects and advantages thereof may bestbe understood by reference to the following description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a graph showing the relative transmission of an idealizedbleachable layer as a function of dosage applied to an entrance surfacethereof.

FIGS. 2A-2F show diagrams useful in explaining the dynamics of thebleaching process used in the present invention.

FIG. 3 is a graph of the relative transmission of a particular contrastenhancing layer as a function of time in response to the application ofan incident dosage of a particular value.

FIG. 4 shows a set of graphs showing relative transmission and contrastof an image having 30% contrast for the case of maximum and minimumtransmission and the instantaneous contrast and integrated contrastproduced thereby as a function of incident dosage.

FIG. 5 shows graphs of relative transmission and contrast as a functionof transmitted dosage for the graphs of FIG. 4.

FIGS. 6A-6E show cross sections of structures representing successivesteps in one method for carrying out the present invention.

A large fraction of optical lithography is done current by projectiontechniques in which an aerial image of a mask is used to expose thephotoresist. For an aerial image of low contrast, even those parts ofthe image that correspond to the dark regions of the mask havesignificant intensities. As the contrast is reduced, discrimination ofthe darker area from the lighter area becomes increasingly difficult. Inaccordance with the present invention a method is provided whichenhances the contrast of the image incident on the photoresist andthereby improves this discrimination. The contrast enhancement is basedon the use of photobleachable materials which are initially relativelyopaque, but following some dose of radiation, become relativelytransparent. The optical tranmission of an idealized bleachable layer isshown in FIG. 1. When the aerial image of a mask is incident on such alayer, the regions of the bleachable layer that are exposed to thehighest intensities bleach through first, while those parts of the layerthat receive the lowest intensitives bleach through at a later time.

The dynamics of this bleaching process are depicted in FIGS. 2A-2F. FIG.2A shows the relative transmission of an object such as mask consistingof opaque regions producing zero transmission 11 separated by open ortransparent regions producing 100% transmission 12. I_(o) is incidentintensity of radiation. I is transmitted intensity of radiation. E_(b)is the dosage of radiation required to cause bleaching of the layer.FIG. 2B shows a graph 13 of the relative intensity as a function ofposition of an aerial image of the mask when placed in opticalprojection apparatus for producing an aerial image used, for example,for exposure of a layer of photoresist. It is assumed that thewavelength of light used and the dimensions of the regions of the maskare such as to produce the contrast shown. I_(max) is the maximumintensity of the image and I_(min) is the maximum intensity of theimage. FIG. 2C shows the cross-section of a layer 15 of an idealizedphotobleachable material. FIG. 2D shows the bleaching of the layer 15 asdelineated by the dotted line 16 at the end of a first period of time.FIG. 2E shows the bleaching of the layer 15 as delineated by dotted line17 at a later period of time. FIG. 2F shows the bleaching of the layer15 as delineated by dotted lines 18 and 19 at the time the exposure isterminated and bleaching stopped. If the exposure is stopped at a timecorresponding to FIG. 2F, the transmission of the bleachable layercorresponds to that of the original mask. When such a material is coatedon top of a conventional photoresist layer, the resulting composite canhave lower contrast threshold than the contrast threshold of resistlayer alone. This will be true if the photoresist is sensitive enough tobe exposed in a time short compared to the bleaching time. Thebleachable layer essentially forms an in situ contact mask for thephotoresist layer. The net effect of this in situ mask is to increasethe contrast which is incident on the photoresist over the contrast ofthe aerial image.

The application of contrast enhancing techniques to submicron opticallithography raises several physical and chemical constraints on thecontrast enhancing layer itself. The contrast enhancing layer must besimultaneously thin and optically dense. The thickness requirementarises because the narrow depth of focus of high resolution opticalsystems. This limits the thickness to a range of less than about 1micron. Because the contrast enhacing layer must be optically dense, itis necessary that the photochemical constituent of the layer be stronglyabsorbing. Since the optical transmission following bleaching isdetermined by the absorption of photoproducts, the photoproducts musthave a much smaller extinction coefficient than the parent molecule.Extinction coefficient is defined by the equation ##EQU1## where A isabsorbance,

b is path length (cm.), and

c is concentration (mole per liter).

Absorbance is defined by equation ##EQU2## where I_(o) is intensity ofincident radiation, and

I is intensity transmitted radiation.

The extinction coefficient ε of a material is obtained by determiningthe parameters A, b and c in equation (1). The parameter A is obtainedfrom equation (2). Initially, a known amount of a material by volume isdissolved in a known amount of a solvent by volume to obtain theconcentration c of the material in the solution. This solution is putinto a cell of known dimensions and placed in the light path of aspectrophotometer and radiation of known intensity I_(o) is directedonto the cell. The intensity of the radiation transmitted from the cellis measured. The solvent alone is also put into another cell of the sameknown dimensions and placed in the light path of the spectrophotometerand radiation of known intensity I_(o) is directed onto the cell. Theintensity of the radiation transmitted from the cell is measured. Theintensity measurements of solvent alone is used to correct thetransmitted intensity I for cell and solvent absorption. Thus usingthese values of incident intensity I_(o) and transmitted intensity I inequation (2), absorbance A is obtained. Dimension b is obtained from thecell of known dimensions. Thus, the extinction coefficient for thematerial is obtained by substitution of values b, c and A in equation(1). The measurement of the extinction coefficients of materials is alsodescribed on pages 644-652 in "Fundamentals of Analytic Chemistry", 2ndEdition 1969) by Douglas A. Skoog and Donald M. West, published by Hold,Rinehart and Winston, Inc. of New York, New York. Continuing, in orderto minimize the necessary increase in exposure time, the quantum yieldof the bleaching reaction must be as high as possible. Also, sincephotoresists are conventionally applied by spin coating techniques, itwould be convenient if the contrast enhancing layer could also beapplied by similar methods. The solvent in which the bleachable materialis dissolved must be compatible with photoresist layers. Further, it isrequired that it be possible to spin coat contrast enhancing layers ofgood optical quality. Finally, the wavelength range over which thesebleachable materials operate must be the same as the wavelength rangeover which the optical projection system operates. Mostdirect-step-on-the-wafer systems operate at 405 nm (nanometers) or 436nm. In this case, the 405 nm wavelength was chosen for use on anOptimetrix 10:1 DSW system available from Optimetrix Co. of MountainView, Calif. The search for appropriate bleaching materials was madewith regard to these constraints. Particularly, the search forappropriate bleaching materials was made with regard to absorptionmaxima in the wavelength range of 300 to 450 nanometers.

A model of the bleaching process was developed and utilized in theevaluation of bleachable materials suitable for use in the contrastenhancing layer. The parameters of the model are set forth in thefollowing table:

                  TABLE 1                                                         ______________________________________                                        Quantity      Explanation                                                     ______________________________________                                        .sup.ε A                                                                            Extinction coefficient of                                                     unbleached molecules                                            .sup.ε B                                                                            Extinction coefficient of                                                     bleached molecules                                              φ         Quantum yield of the                                                          bleaching reaction                                              N.sub.o       Initial density of unbleached                                                 molecules                                                       F.sub.o       Flux density of photons                                                       incident on contrast enhanc-                                                  ing layer                                                       t.sub.o       Thickness of contrast enhanc-                                                 ing layer                                                       n.sub.s       Index of refraction of                                                        bleached layer                                                  n.sub.s       Index of refraction of glass                                                  substrate on which the con-                                                   trast enhancing layer is                                                      situated                                                        ______________________________________                                    

Based on the analysis above and the model of the bleaching process threecriteria for material parameters were developed and are set forth inTable 2.

                  TABLE 2                                                         ______________________________________                                        Quantity            Value                                                     ______________________________________                                        (1)   ε/Molecular Weight                                                                      ≧100 liters/gram-cm                            (2)   Φ             ≧0.2                                           (3)                                                                                  ##STR1##         ≧30                                            ______________________________________                                    

The first criterion is based on the need for an optically dense film,and is essentially related to the packing density of absorbing centersin the contrast enhancing layer. The second criterion is based on theneed for as abrupt a transition from the unbleached to the bleachedstate as is possible. Acceptability of a given quantum yield is to someextent related to the first criterion, because improvement in the firstcan compensate for deficiencies in the second criterion. The thirdcriterion is based on the need for the contrast enhancing layer to betransparent following the bleaching process. These criteria were used inthe initial search for appropriate bleachable materials.

Selection of suitable bleachable compound was determined by evaluationin the model of the bleaching process and by test of the compound in alayer thereof to determine the relative transmission as a function oftime or dosage of radiation with radiation intensity being heldconstant. A number of different bleachable compounds, the bleachingproperties of which were based on different bleaching mechanisms wereevaluated. The bleachable compounds dependent on photoisomerization werefound to be particularly suitable. Of these compounds, aryl nitronesrepresented by formula (1), were found particularly suitable. ##STR2##In formula (1), Z is a monovalent group selected from (R³)_(a) --Q--R⁴-- or R⁵ -- and Z' is a monovalent group selected from --R⁶ (X)_(b), R,R¹, R² and R³ are monovalent radicals selected from the class ofhydrogen, C.sub.(1-8) alkyl, C.sub.(1-8) substituted alkyl, C.sub.(6-13)aryl hydrocarbon and C.sub.(6-13) substituted aryl hydrocarbons. Q is amonovalent, divalent or trivalent atom selected from the group F, C, Br,I, O, S, N, where a can have values of 0, 1 or 2. R⁴ is a C.sub.(6-13)aryl hydrocarbon or a C.sub.(6-13) substituted aryl hydrocarbon. R⁵ isselectable from the group of substituted or unsubstituted C.sub.(6-20)aromatic heterocyclic compounds incorporating one or more atoms from thegroup O, N or S. R⁶ is selected from the group of C.sub.(6-20) aromatichydrocarbons and X is selected from the group of halo, cyano, alkylcarbonyl, C.sub.(1-8) alkyl, C.sub.(1-8) substituted alkyl, C.sub.(6-13)aryl hydrocarbon, C.sub.(6-13) substituted aryl hydrocarbons, or alkoxycarbonyl in any combination for values of b of 0, 1, 2 or 3, n can havevalues of 0, 1, 2, 3 or 4. The above compounds can be prepared usingprocedures such as those described in "Methoden der Organischen Chemie(Houben-Weyl), Vol. 10, part 4 (1968), pgs. 315-416, or those describedin Chemical Reviews (1964), NItrones, by Jan Hamer and Anthony Macaluso,pgs, 476-483.

Various aryl ring systems with a variety of substituents may beconstructed to suit the particular needs of the optical system employedin the photoimaging process. The aryl nitrones exhibit extinctioncoefficients of 2 to 5×10⁴ liter mole⁻¹ cm⁻¹ and bleach with quantumyields in the range of 0.1 to 0.5.

For direct-step-on-the-wafer systems capable of imaging at 405 nm, thenitrones of the general structure (2) were found to be particularlyuseful. R1 ? ##STR3## where Ar is a C.sub.(6-14) aromatic organicradical. Included among this subclass of p-dialkylaminoaryl nitrones areheterocyclic compounds such as (3). ##STR4##

A preferred class of nitrones shown by formula (2) are diaryl nitronesof the formula ##STR5## where X is an electron withdrawing group in theortho or para position selected from the class consisting of ##STR6##where R⁷ is a C.sub.(1-8) alkyl radical and more particularly methyl,ethyl, propyl and butyl. Some of the halogen radicals included by X, arefor example, fluoro, chloro and bromo.

Suitable binders for use in providing a spin castable mixture forformation of a photobleachable layer incorporating the aryl nitrones offormula (1) are: vinyl acetate polymers (homopolymers and copolymers)and their partially saponified products (e.g., polyvinylacetate),copolymers of styrene or its derivatives, polymers and copolymers ofacrylate or methacrylate esters, acetal resins, acrylonitrile/butadienecopolymers, ethyl cellulose and other hydrocarbon-soluble celluloseethers, cellulose propionate and other hydrocarbon soluble celluloseesters, poly(chloroprene), poly(ethylene oxide), poly(vinylpyrrolidone).

Suitable solvents for use in providing a spin castable mixture forformation of a photobleachable layer incorporating the aryl nitrone offormula (1) are: aromatic hydrocarbons (e.g. toluene xylenes, ethylbenzene, chlorobenzene) with or without aliphatic hydrocarbons (e.g.cyclohexane), halogenated aliphatic compounds (e.g. trichloroethylene,methyl chloroform, alcohols (e.g. propanol, butanol).

The diaryl nitrone 2 where R³ is --CH₃ CH₂ and n=0 was found to beparticularly suitable. This nitrone, referred to asα-(4-diethylaminophenyl)-N-phenylnitrone, was found to absorb stronglyat 405 nm and bleaches to near transparency with high efficiency at thesame wavelength by undergoing unimolecular cyclization to anoxaziridine. The nitrone is very soluble in solvents of moderately lowpolarity (e.g., toluene, ethylbenzene) and forms good films at highloading densities with a variety of polymers such as polystyrene,poly(hydroxyethylmethacrylate), poly-α-methylstyrene,poly(methylmethacrylate), polyvinylpyrrolidone, vinylpyridine/styrenecopolymers and allyl alcohol/styrene copolymers. The materialα-(4-diethylaminophenyl-N-phenylnitrone has an extinction coefficient toweight ratio of 130 liters/gram-cm at 405 nm. The material was formedinto a contrast enhancing layer as follows: A solution ofα-(4-diethylaminophenyl-N-phenylnitrone (5% by weight of solution) and abinder, styrene/allyl alcohol copolymer (5% by weight of solution) aredissolved in toluene. A glass substrate is spin coated to a thickness of250 nm. The relative transmission of the sample was tested at 405 nm andwas found to have the relative transmission versus time characteristicat 405 nm, shown in FIG. 3.

The model of the bleaching process was used to calculate theimprovements in contrast as a function of exposure time for the contrastenhancing layer of FIG. 3. This is accomplished by calculating thebleaching for two representative points in a given pattern. In thisexample, two incident intensities that correspond to the maxima andminima of line and space grating pattern are chosen. The contrast C thatthese two levels of intensity correspond to can be calculated from thedefinition of contrast: ##EQU3## Using the model of the bleachingprocess, the transmitted intensity as a function of incident dose forboth the maxima and minima are determined. From these quantities, boththe instantaneous and the integrated contrast can be calculated as afunction of incident dosage. FIG. 4 shows graphs of such determinationsand calculations for a 30% contrast pattern incident on the layer of thesample described above. Graph 21 shows the relative transmission at arelative maximum as a function of incident dosages in joules. Graph 22shows the relative transmission at a minimum as a function of incidentdosage. Graph 23 shows the instantaneous contrast obtained from graphs21 and 22 using equation 3, as a function of incident dosage. Graph 24shows the integrated contrast obtained from equation 3 using integratedvalues of I_(max) and I_(min) instead of instantaneous values. Sincebleaching is a dynamic process, the contrast which results from thetransmitted radiation is also a function of dosage. A more usefulgraphical representation is shown in FIG. 5 wherein the correspondinggraphs are plotted as a function of transmitted dosage. Thisrepresentation allows the degree of contrast enhancement to be estimatedas a function of the sensitivity of photoresist to be used in connectionwith the contrast enhancing layer. Graph 26 shows relative transmissionat a relative maximum as a function of transmitted dosage. Graph 27shows relative transmission at a relative minimum as a function oftransmitted dosage. Graph 28 shows the instantaneous contrast obtainedfrom graphs 26 and 27 using equation 3 as a function of transmitteddosage. Graph 29 shows the integrated contrast obtained from equation 3using integrated values of I_(max) and I_(min), as a function oftransmitted dosage.

The process used for utilizing the contrast enhancing layer will now bedescribed and thereafter the results obtained thereby will be comparedwith the results obtained under the same conditions but without the useof a contrast enhancing layer. Reference is now made to FIGS. 6A-6Ewhich illustrate the various steps of the process for providing apattern of photoresist on a suitable substrate. FIG. 6A shows asubstrate 31 on which is provided a layer 32 of a suitable photoresistsuch as Shipley 1400 series of positive photoresists available from theShipley Company of Newton, Mass. Such positive resists consist of anovolac resin or poly(vinylphenol), diazonaphthoquinone esters, andsolvents (e.g. cellosolve acetate, xylenes). The liquid photoresist isdeposited on the surface of the substrate which is then spun to providea layer of desired thickness. After baking of the phootoresist layer toremove solvent, a contrast enhancing layer 33 is provided on the surfaceof the photoresist. The contrasting enhancing layer 33 is constituted ofa solution of a styrene/allyl alcohol copolymer binder (5% by weight ofsolution) and α-(4diethylaminophenyl)-N-phenylnitrone (5% by weight ofsolution) in the solvent toluene. The solution is deposited on thesurface of the photoresist 32, spun and thereafter baked to removesolvent to provide the layer 33 of desired thickness, as shown in FIG.6B. The resultant structure is then exposed to a pattern of radiationconsisting of illuminated regions 35 underlying the arrows 37 indicatingradiation and non-illuminated regions 36 for a time to produce acontrast enhanced image in the range of dosages to which the photoresistlayer is sensitive and which fully expose the layer of photoresist, asshown in FIG. 6C. Thereafter, the contrast enhancing layer 33 is removedby using a suitable stripping solvent, such as trichloroethylene, whichremoves the contrast enhancing layer without affecting the photoresist,as shown in FIG. 6D. Thereafter, the exposed portions of the photoresistare removed leaving retaining portions 38, 39 and 40 which are unexposedor insufficiently exposed as shown in FIG. 6E.

In order to determine the improvements in contrast threshold obtainedfor the composite layer of the contrast enhancing layer and photoresistin accordance with the present invention, the aforementionedphotobleachable material constituted ofα-(4-diethylaminophenyl)-N-phenylnitrone and a styrene/allyl alcoholbinder was utilized in conjunction with Shipley 1400 series of positivephotoresist to fabricate various patterns. A wafer or substrate ofsilicon was coated with only photoresist having a thickness of 1.6microns and another wafer of silicon was coated with a layer ofphotoresist 1.6 microns thick over which was coated a layer of thephotobleachable material specified above to a thickness of 0.25 microns.An object consisting of an opaque line 2 micron wide, a transparentspace 2 microns wide, an opaque line 0.8 microns wide and a transparentspace 0.8 microns wide was imaged using the Optimetrix 10:1 projectionsystem at 405 nanometers onto the wafer with just the layer ofphotoresist with a range of dosages to form a number of patterns in thephotoresist and was also imaged on the wafer which included photoresistand contrast enhancing layer with a range of dosages to form a number ofpatterns in the photoresist. The photoresist on each of the wafers wasdeveloped to obtain the patterns of lines and spaces formed in thephotoresist. Patterns produced in the wafer with just photoresist usingthe minimum exposure of dosage that opened the 0.8 micron space at thephotoresist substrate interface were compared with the patterns producedin the wafer with photoresist and contrast enhancing layer using theminimum exposure or dosage that opened 0.8 microns space at thephotoresist substrate interface. The 2.0 micron wide line and the 2.0micron wide space, were nearly correct for both the wafer using just thephotoresist and the wafer using the photoresist and the contrastenhancing layer thereon. The 0.8 micron wide line was grosslyoverexposed on the wafer having just the photoresist using the exposurewhich produced a 0.8 micron wide space in the photoresist. The 0.8micron wide line was properly exposed producing a 0.8μ wide line in thephotoresist on the wafer having both photoresist and contrast enhancinglayer using the exposure which produced a 0.8μ wide space in thephotoresist. Furthermore, the profiles of the walls of the contrastenhanced patterns were nearly vertical. The poorer resultant patternsproduced in the photoresist without the contrast enhancing layer is dueto the much lower aerial image contrast obtained at the output of theOptometrix projection system due to the higher spatial frequenciespresent in the image or pattern of radiation obtained therefrom.

While the invention has been described in connection with a particularpositive photoresist, other positive and negative photoresists may beutilized. Also, while a particular thickness of photoresist and aparticular thickness contrast enhancing layer were utilized in oneexample describing the invention, it will be understood that otherthicknesses of photoresist and contrast enhancing layers may beutilized. Preferably the layer of photoresist should have a thicknessless than about 3 microns and the contrast enhancing layer should have athickness less than about 1 micron.

While in an exemplary composition of the contrast enhancing layer equalweight proportions of the photobleachable compound and the bindertherefor were utilized, other proportions may be utilized, if desired.

While in connection with photoresists it was mentioned that they have acharacteristic called contrast threshold, it should be noted that thischaracteristic is a function of conditions of usage of the photoresistas well as of the resist itself, for example, the nature of thesubstrate on which used and reflections therefrom.

While values of the extinction coefficient to molecular weight ratio inthe unbleached state for the photobleachable compound greater than about100 are preferred and a ratio of the extinction coefficient for theunbleached state to the extinction coefficient for the bleached state ofthe photobleachable compound greater than about 30 is preferred, valuesof the above ratios as low as about 10 would be satisfactory.

While a particular class of photobleachable compounds, namely, the arylnitrones, dependent on unimolecular cyclization were utilized, it willbe understood that other photobleachable compounds dependent onunimolecular cyclization and on other bleaching mechanisms such asphotofragmentation, for example, may be utilized in accordance with thepresent invention.

The aforementioned α-(4-diethylaminophenyl)-N-phenylnitrone, also setforth in Table I, was prepared by condensing p-diethylaminobenzaldehyde(18.5 g, 0.1 mole) with freshly prepared phenylhydroxylamine (11.4 g,0.1 mole) in 40 ml of absolute ethanol at room temperature for 18 hours.Evaporation of the solvent yielded a red oil which was twicecrystallized from toluene/petroleum ether to afford 13.0 g (0.05 mole)of the nitrone, mp 103°-105° C. Further recrystallization of ananalytical sample raised the melting point to 110°-112° C.

Other nitrones having absorption maxima in the wavelength rate of 300 to450 nanometers are also set forth in Table I. In Table I, λ_(max) (nm)designates absorption maximum in nanometers, ε_(max) designatesextinction coefficients at wavelength of maximum absorption, mpdesignates melting point in degrees Centigrade.

These other nitrones wee prepared in a manner similar to the manner ofpreparation to the aforementioned nitrones by the condensation of theappropriate aldehydes and phenylhydroxylamines in polar solvents.

                                      TABLE 1                                     __________________________________________________________________________     ##STR7##                                                                     Ar               Ar'           λ.sub.max (nm)                                                              ε.sub.max                                                                 mp (°C.)                       __________________________________________________________________________    α-(4-diethylaminophenyl)-N-phenylnitrone                                (1)                                                                               ##STR8##                                                                                    ##STR9##     388  41,000                                                                            110-11                                α-(4-diethylaminophenyl)-N-(4-chlorophenyl)nitrone                      (2)                                                                               ##STR10##                                                                                   ##STR11##    389  42,500                                                                            176-179                               α-(4-diethylaminophenyl)-N-(3,4-dichlorophenyl)nitrone                  (3)                                                                               ##STR12##                                                                                   ##STR13##    409  43,700                                                                            154-157                               α-(4-diethylaminophenyl)-N-(4-ethoxycarbonylphenyl)nitrone              (4)                                                                               ##STR14##                                                                                   ##STR15##    418  31,300                                                                            107-109                               α-(4-diethylaminophenyl)-N-(4-acetylphenyl)nitrone                      (5)                                                                               ##STR16##                                                                                   ##STR17##    424  27,000                                                                            115                                   α-(4-diemthylaminophenyl-N-(4-cyanophenyl)nitrone                       (6)                                                                               ##STR18##                                                                                   ##STR19##    429  33,000                                                                            202-203                               α-(4-methoxyphenyl)-N-(4-cyanophenyl)nitrone                            (7)                                                                               ##STR20##                                                                                   ##STR21##    368  13,00                                                                             181-182                               α-(9-julolidinyl)-N-phenylnitrone                                       (8)                                                                               ##STR22##                                                                                   ##STR23##    405  38,000                                                                            132-133                               α-(9-julolidinyl)-N-(4-chlorophenyl)nitrone                             (9)                                                                               ##STR24##                                                                                   ##STR25##    420  36,600                                                                             99-103                               α-[2-(1,1-diphenylethenyl)]-N-phenylnitrone                             (10)                                                                              ##STR26##                                                                                   ##STR27##    366  27,800                                                                            140-142                               α-[2-(1-phenylpropenyl)]-N-phenylnitrone                                (11)                                                                              ##STR28##                                                                                   ##STR29##    340  19,000                                                                            468                                   __________________________________________________________________________

In the method of the invention the photobleachable layer is in the formof an in situ mask on the layer of photoresist. The formation of such acomposite structure has a number of advantages. The photobleachablelayer conforms to the surface of the layer of photoresist and avoids theformation of gaps between the high points of the surfaces of the layerof photoresist and the layer of photobleachable material. Such gapswould be formed when the two layers are formed on separate supports andthen brought together. Such gaps would be highly detrimental to theresolution of any image formed in the photoresist particularly when thefeatures to be imaged are of the order of a few microns and also whenthe depth of field of the projection system is a few microns. Also,bring a layer of photobleachable material into contact with a layer ofphotoresist and thereafter separating them runs the risk of havingpieces of one layer adhering to the other layer thereby damaging thelayers of photoresist.

While in describing the invention in an exemplary embodiment, thecontrast enhancing layer was situated in contact with the layer of thephotoresist, the contrast enhancing layer could have been spaced apartfrom the layer of the photoresist, if desired, for example, by a thinconformal layer of neutral material formed in situ.

While the invention has been described in specific embodiments, it willbe understood that modifications such as those described above may bemade by those skilled in the art, and it is intended by the appendedclaims to cover all such modifications and changes as fall within thetrue spirit and scope of the invention.

What is claimed is:
 1. The compoundα-(4-diethylaminophenyl)-N-phenylnitrone.